Re: [CR] twin-plated fork crowns

(Example: Framebuilders:Chris Pauley)

In-Reply-To: <20050125144012.44279.qmail@web30502.mail.mud.yahoo.com>
References: <20050125144012.44279.qmail@web30502.mail.mud.yahoo.com>
Date: Tue, 25 Jan 2005 07:52:34 -0800
To: classicrendezvous@bikelist.org
From: "Jan Heine" <heine93@earthlink.net>
Subject: Re: [CR] twin-plated fork crowns


I was just pointing out that with brazed-on brake pivots, the fork blades and fork crown become part of the brake. Even when stationary, as soon as you squeeze the brake, the forces are transmitted through the fork, twisting the blades (because the braze-ons are on the front (or back), off the center axis of the blade). (This force is different from the one that always will "push your front wheel backward" as you brake the front wheel when in motion, no matter the type of brake.)

Imagine fixing the frame and front wheel is some sort of holder, so things don't fall apart during the next step.* Then remove the entire fork crown, but leave the steerer tube, so that your sidepull brake still can be attached with the centerbolt.

- A brake with a center bolt (sidepull, standard centerpull) still can be squeezed and the brake pads will press against the rim. The fork legs will not move during this operation. (When stationary, the fork crown and legs do not form part of the braking system).

- A brake with brazed-on pivots will move the fork legs outward while squeezing the rim. This additional stress is imparted by brazed-on brake pivots. (In fact, that is the whole theory behind using brazed-on pivots on centerpulls: Rather than using the crescent-shaped piece connecting the pivots, which basically duplicates the upper fork legs and crown, you just use the fork legs and crown themselves to transmit those forces.)
>Is anyone seriously asserting that a _properly built_ twin
>plate fork crown is weaker than other types?
>
> Best regards,
> Fred Rednor - Arlington, Virginia
>

Not at all. But it must be _properly designed_ for the stresses involved. Putting brake braze-ons on your Masi GC to convert it to cyclocross may have unintended consequences! (This is a joke, nobody would do something like that, of course.) But a lot of builders on this list are incorporating various historical elements into their new KOF frames (a very good thing, IMHO), so I thought I'd point out something that has caused problems in the past. A broken fork crown is no joke. Considering that hundreds of these fork crowns have given total satisfaction, I would not consider this a poorly designed crown. It just wasn't designed for this particular application.

Jan Heine, Seattle

*You'll also need to block the front hub...